The present invention relates to the testing of low voltage power circuit breakers.
The basic design of the low voltage power circuit breaker has been utilized in virtually all large industrial facilities since the 1930's. The circuit breakers of that vintage and those manufactured up to the late 1960's exclusively utilized electromechanical overloads which sensed current and provided a time delay in the case of an over-current or fault. Later versions utilized electronic controls, first solid state and then microprocessor-based.
As used herein, the term “electronic trip unit” means any version of electronic trip devices, either solid state or microprocessor controlled, which can be used to assist in the testing. The electronic trip unit controls afford many advantages, including more versatility in settings and the ability to trip the breaker due to ground faults. Moreover, some microprocessor-controlled trip units offer advanced features such as communications and logging of trip counts and trip data.
Due to the critical applications in which circuit breakers may be used, they are always tested prior to being placed into service and during scheduled preventive maintenance intervals. These tests are designed to verify the functionality of the breaker's mechanical components and of the electronic trip unit.
Standard industry practice is to use one of two basic testing methods to verify the operation of the breaker and electronic trip unit. Both of these test methods are typically performed on a single phase and are repeated for each of the three breaker phases. Primary injection involves passing a low voltage, high current test signal through the primary winding of the current sensor which is interfaced directly to the electronic trip unit. Secondary injection testing involves injecting a low voltage, low current directly into the electronic trip unit.
In order to test a breaker using the current technology, the test technician is faced with several obstacles. These include:                1. It is not possible to test certain functions without disabling others. An example of a function that can not be tested is the instantaneous pickup point. Often, a trip device is programmed with two protection bands which are designed to protect against short circuits. One band is commonly known as the “short-time protection band”. This band provides a programmable delay of 0.07 to 0.500 seconds before tripping the breaker after a preset current threshold is exceeded. The other band is commonly known as “instantaneous protection” and trips a breaker immediately if a current threshold is exceeded. Typically, the short-time threshold or “pickup point” is set lower than the instantaneous pickup point. These pickup points are tested by slowly ramping up a test current and verifying that the unit under test trips when the current is between preset tolerance limits. Testing the instantaneous pickup with short-time protection is not possible because the electronic trip unit will trip the breaker based on the short-time setting before the instantaneous pickup-point is reached.        2. Low voltage power circuit breakers are typically tested by injecting a single phase of current, either through the sensor primary or directly into the electronic trip unit. By definition, protection elements such as ground fault and phase imbalance will cause a breaker to trip when it senses a single phase. This prohibits other functions from being tested.                    Prior to this invention, two options were available to temporarily disable these functions. First, settings could be manually disabled by use of the setup controls on the electronic trip unit. This task is tedious, time consuming and susceptible to human error in that these settings must be correctly re-set before the breaker is returned to service. Second, a hardware device known as a “ground defeat cable” can be used to allow testing on a breaker that has an electronic trip unit with ground fault protection. The ground defeat cable is inserted between the electronic trip unit and the breaker-mounted current sensors. A disadvantage in using a ground defeat cable is that the breaker wiring harness must be disconnected from the electronic trip unit. Often, due to space limitations, it is very difficult to access this connector and some disassembly can be required. More seriously, when the ground defeat cable is removed and the breaker is readied to be returned to service, the current sensor must be properly re-connected to the electronic trip unit. Because of space limitations, it is often difficult to properly connect this cable or to verify the integrity of the connection. If an improper electrical connection between the current sensors and the electronic trip unit is made, the current sensors will be “open circuited” upon re-energization. Open circuited current sensors generate a high voltage which can be dangerous to personnel and can damage wiring, the electronic trip unit and the current sensors themselves.                        3. Because interrupting by a high current fault has adverse effects on the service life of the breaker as well as the primary injection test set, breakers are often tested with settings temporarily lowered. With present technology, the test technician must manually adjust the settings on the electronic trip unit. If changes are made to the electronic trip unit settings, there is a possibility that these settings will not be properly restored prior to the breaker's being returned to service.        4. Because the menu systems in all presently available trip units are designed for ease of initial setup, not for temporarily changing settings or disabling functions, the test technician is required to scroll through a complete setup routine to make these temporary changes. This process is both time consuming and prone to human error due to the fact that the technician may incorrectly restore the original settings or omit performing this task altogether.        5. Some microprocessor based trip units have features that log the details of trips and/or count the number of trips that have occurred. It is not desirable to log trip events that occur during testing. Using present technology, the operator is forced to either clear the entire trip history or to retain the trips that were logged during a test sequence.        6. Some microprocessor based trip units have auxiliary outputs that communicate information such as the status of alarm contacts or tripping restraint signals to other electronic trip units or to outside systems. These features are typically used in control or automation or zone-interlocking schemes. Because these status indicators become active only when specific conditions occur and by definition involve interaction between multiple trip units or between trip units and outside systems, testing using existing methods is difficult and time consuming.        Power circuit breakers are often used in applications where they are required to continually be in service. Examples of these applications include data centers, hospitals and airports. Opportunities to perform maintenance and testing are rare and the durations of scheduled outages are limited.                    Since minimizing the duration of an outage is critical, test technicians are required to work long shifts with minimal breaks. Often, several hours of preparation precede the beginning of an outage. The possibility of human error in these instances is high due to the stress and fatigue.            Accordingly, it is an object of the present invention to provide a novel method for testing low voltage power circuit breakers.            It is also an object to provide such a method which lessens the time required for the testing of a circuit breaker.            Another object is to provide such a method which precludes inadvertent resetting of the breaker at other than the original settings before testing.                        